Add code to set TCP keep alive delay on OSX.

Previously, TCP keep alives were enabled on all
desktop platforms, but the delay was not being set on
OSX, resulting in not sending keep alives every 45
seconds, like on other platforms.  This CL adds the
code to do that.

It does the same on iOS, though keep alives aren't
enabled on mobile platforms, anyways.

BUG=468764

Review URL: https://codereview.chromium.org/1065563006

Cr-Commit-Position: refs/heads/master@{#324319}

net/socket/tcp_socket_libevent.cc

@@ -56,6 +56,7 @@ bool SetTCPNoDelay(int fd, bool no_delay) {
 
 // SetTCPKeepAlive sets SO_KEEPALIVE.
 bool SetTCPKeepAlive(int fd, bool enable, int delay) {
+  // Enabling TCP keepalives is the same on all platforms.
   int on = enable ? 1 : 0;
   if (setsockopt(fd, SOL_SOCKET, SO_KEEPALIVE, &on, sizeof(on))) {
     PLOG(ERROR) << "Failed to set SO_KEEPALIVE on fd: " << fd;
@@ -67,6 +68,8 @@ bool SetTCPKeepAlive(int fd, bool enable, int delay) {
     return true;
 
 #if defined(OS_LINUX) || defined(OS_ANDROID)
+  // Setting the keepalive interval varies by platform.
+
   // Set seconds until first TCP keep alive.
   if (setsockopt(fd, SOL_TCP, TCP_KEEPIDLE, &delay, sizeof(delay))) {
     PLOG(ERROR) << "Failed to set TCP_KEEPIDLE on fd: " << fd;
@@ -77,6 +80,11 @@ bool SetTCPKeepAlive(int fd, bool enable, int delay) {
     PLOG(ERROR) << "Failed to set TCP_KEEPINTVL on fd: " << fd;
     return false;
   }
+#elif defined(OS_MACOSX) || defined(OS_IOS)
+  if (setsockopt(fd, IPPROTO_TCP, TCP_KEEPALIVE, &delay, sizeof(delay))) {
+    PLOG(ERROR) << "Failed to set TCP_KEEPALIVE on fd: " << fd;
+    return false;
+  }
 #endif
   return true;
 }